Previously, it has been proposed to use gel chromatography to separate platinum group metals (PGM) from one another on an industrial scale. Prior proposals include U.S. Pat. No. 4,885,143 (Schmuckler). This patent describes a method in which the interseparation of platinum group metals (PGM) from an oxidised gold-free halide solution is achieved using a chromatographic medium such as a polysaccharide gel (Sephadex) or a polyacrylamide gel (Biogel). The PGM when dissolved in a chloride solution are absorbed onto the chromatographic column and are claimed to be selectively eluted in the order ruthenium, rhodium, palladium, platinum, iridium and caesium, although it is clear from the rest of the patent that Schmuckler meant osmium rather than caesium. The problem with this method is that there is in fact no clear separation of PGM.
This problem was to a large extent overcome by European patent application EP 756013 (Matthey Rustenburg Refiners Pty) which describes a method for the interseparation of PGM from a PGM-containing halide solution comprising the steps of passing the solution through a glycol methacrylate chromatographic medium, absorbing the PGM onto the medium, eluting each PGM using an acid solution to obtain each fraction containing at least one PGM. During the elution of a reduced, mixed rhodium, iridium, ruthenium, palladium, platinum and osmium/6 molar hydrochloric acid solution through Toyopearl HW-40C using a (usually 6 M) hydrochloric acid eluent, the first eluted band contains trivalent rhodium, iridium and ruthenium. It is clear that the method does not separate rhodium, iridium and ruthenium; either from each other or any combinations/permutations within. Rhodium, iridium and ruthenium are sometimes referred to as the insoluble metals. Furthermore, rhodium is conventionally difficult to chromatographically separate from a mixed solution with one or more base metals such as nickel and copper.
EP 1167555 A1 describes a process using continuous annular chromatography for the interseparation of base metals and platinum group metals. Two distinct media are required. The first comprises a cation exchange resin which is used to absorb the base metals, and the second is a size exclusion gel which is used to effect a separation of the PGM. The process is further complicated by the need for an inert layer to be present between the two media in order to prevent intercontamination. The purity of the PGM fractions obtained is also influenced by the concentration of the eluent and of the feed solution. Furthermore, the process does not separate ruthenium from rhodium.